The technical answer is to make them as robust as possible following M. Brown's suggestions (she's very good!), PLUS hold them indoors at a reasonable temperature (no hotter than the seventies) until the last iminute. If you can't keep them indoors, don't bother.
However, the more general and more importat answer is that cream puffs are a poor choice for the weather. Not so much because it's hard to get them to hold up to it, but because they're just too rich and will make your guests feel overstuffed and guilty.
One (among many) good ways to think about desserts is to divide them into chocolates, fruits and lemons (which includes anything citrussy), and others (vanillas, nuts and mints by way of a few examples). A lemon would be ideal for the weather, but because it's a wedding (strong romantic demographic) you probably want at least one chocolate.
If you absolutely must have something creamy, think about something which may be held on ice and served cold. For instance a selection of mousses, is probably your best bet. The mousses could (and should) be garnished with whipped cream at the time of service. The choice might be Mexican Chocolate with rum and Limoncello (for the adults), and a non-alchoholic butterscotch or praline mousse for the kids and teetotalers. By way of a pastry, you could side with a light cookie such as a macaroon.
Fruit tartes and pies are also robust, and somewhat less fussy to hold and serve. While American pies may seem a bit informal, tarte Tatin is an elegant choice. In fact, it was one of my desserts when I catered my own outdoor wedding recpetion many years ago.
If you want to stay with something rich, you might consider baba au rhum, garnished with a choice of white chocolate (dust with cocoa) or vanilla (plant a mint leaf) whipped creams.
No matter what, strawberries -- especially chocolate dipped -- are occasion appropriate garnishes.
It might be best if you made your choice based on the menu itself. For instance, a strongly regional menu would certainly suggest a similarly regional dessert.
A question for you: How are you planning on working additional desserts into the menu around the wedding cake?